Magnesium alloy



nited States MAGNESIUM ALLOY No Drawing. Application October 5, 1953, Serial No. 384,340

1 Claim. (Cl. 75-168) The invention relates to magnesium alloys. It more particularly concerns magnesium-base alloys containing thorium and having among other desirable properties a high creep strength.

Previous attempts to make a magnesium-base alloy having a high resistance to creep at elevated temperatures leave much to be desired. In one approach to the problem of producing a magnesium-base alloy having a high creep strength, attempts have been made to modify the creep strength of magnesium-base alloys containing as alloying constituents 0.5 to 5 per cent of zinc together with 0.1 to 0.9 per cent of zirconium by the addition of thorium in amount which is not greater than nine-tenths of the zinc percentage plus 2% per cent, i. e. if the zinc is 2 per cent the thorium will not exceed 4.55 per cent (quotation from U. S. Patent 2,604,396). However, such thorium-containing magnesium-base alloys are costly and ditficult to prepare free from oxide inclusion, I have found, and furthermore the resistance to corrosion by salt water is relatively poor. In view of these drawbacks, the use of thorium to enhance the creep strength of the magnesium-base alloys containing zinc and zirconium has not progressed to the point of wide acceptance. I have now discovered that a magnesium-base alloy having both the desirable high creep strength and resistance to salt water corrosion is obtained on alloying together with magnesium only a small amount of each of the metals thorium, zinc, and zirconium.

The actual proportions of the alloying ingredients in the alloy, particularly thorium and zinc, are quite critical. Thus, in accordance with the invention, the amount of thorium is restricted to 0.25 to 0.65 per cent, the zinc content is restricted to between 0.25 and 0.75 per cent, while the zirconium content is between 0.4 and 1.0 per cent. The balance of the alloy is magnesium, either pure magnesium (e. g. sublimed magnesium) or any of the usual commercial grades of magnesium, such as electrolytic magnesium.

The alloy may be prepared by melting a suitable quantity of commercial magnesium in a clean steel melting pot under the protection of a suitable saline flux, such as one free from magnesium chloride. A suitable flux, for example, is a mixture of potassium chloride, calcium chloride, and barium chloride together with some calcium fluoride. The thorium is added to the melt, preferably as a magnesium-thorium hardener, the melt being stirred until the hardener dissolves. If desired, the magnesium and the magnesium-thorium hardener can be melted simultaneously. The zirconium also may be added as a magnesium-zirconium hardener. A convenient zirconium hardener to use comprises 50 per cent Zr and 50 per cent Mg. The zinc is added preferably as metallic zinc. In preparing the alloy by the addition of the foregoing metallic ingredients to the melt of magnesium, it is preferable to operate at about 1350 to 1400 F. Samples of the alloyed melt may be taken for analysis during the alloying operations as a check on the composi- 2,703,753 Patented Mar. 8, 1955 tion obtained. Pursuant to analyses of the melt, adjustments in the proportions of the alloying constituents may be made, if necessary, so that the desired proportions are obtained. The zirconium to be effective must become dissolved in the melt rather flJan merely suspended or dispersed as discrete particles of unalloyed metallic zirconium. The percentage figures given for the proportions of zirconium refer'to dissolved zirconium as distinguished from unalloyed zirconium as understood in the art. A convenient test for alloyed zirconium as distinguished from unalloyed zirconium is to subject the sample of the alloy to the dissolving action of dilute hydrochloric acid (about 15-20 per cent HCl, balance water).

In this test alloyed zirconium dissolves in the acid, where- Example A charge of 20.875 pounds of commercial magnesium was melted in a clean steel crucible in the presence of about one pound of saline flux composed of 57 parts of KCl, 28 parts of CaClz, 12.5 parts of BaClz and 2 /2 parts of CaFz. The melt was heated to 1400 F. At this temperature, 2.125 pounds of a magnesium-thorium hardener were added and the melt was stirred gently until the hardener was dissolved. This required about 1 minute. The magnesium-thorium hardener contained 5.75 per cent of thorium, the balance being magnesium. Zirconium was added as a magnesium-zirconium hardener comprising 50 per cent of zirconium, 40 per cent of magnesium, and 10 per cent of magnesium chloride. The melt was stirred until the metallic portion of the zirconium hardener was dissolved. This required but a few minutes. The zinc was added (56 grams) and the melt stirred a few minutes until the zinc dissolved. The resulting melt was held in a quiescent state for 20 minutes cooled to 1350 F. and then test bars were cast in a green sand mold. The resulting cast alloy had the composition of 0.5 per cent Th, 0.55 per cent Zn, 0.58 per cent Zr (soluble in hydrochloric acid), the balance being magnesium. The cast bars were substantially free from folded oxide skins and oxide inclusions. At room temperature, the as-cast test bars had a tensile yield strength of 10,600 p. s. i. with an elongation of 18.5 per cent and an ultimate tensile strength of 27,900 p. s. i. At 650 F., the test bars had a tensile yield strength of 4500 p. s. i. with an elongation of 30.5 per cent and an ultimate tensile strength of 7700 p. s. i. The corrosion rate by the conventional 14 day alternate immersion test in 3 per cent salt solution was 0.26 milligram per square centimeter per day. The stress, which at 650 F. in hours produced a total extension of 0.2 per cent, was 2760 p. s. i.

Among the advantages of the invention are that a magnesium alloy is obtained which is substantially as light in weight as magnesium itself; the resistance to corrosion in salt water is great; a high resistance to creep is attained; and the amount of alloying ingredients is very small. Wrought metal articles may be produced from the cast alloy by forging, rolling, or extrusion at temperatures between about 650 and 900 F.

I claim:

A magnesium-base alloy consisting of magnesium having alloyed therewith from 0.25 to 0.65 per cent of thorium, from 0.25 to 0.75 per cent of zinc and from 0.4 to 1 per cent of zirconium, the zirconium in said alloy dissolving in hydrochloric acid on subjecting a sample of the alloy to the dissolving action of a 20 per cent solution of aqueous hydrochloric acid.

No references cited. 

1. A MAGNESIUM-BASE ALLOY CONSISTING OF MAGNESIUM HAVING ALLOYED THEREWITH FROM 0.25 TO 0.65 PER CENT OF THORIUM, FROM 0.25 TO 0.75 PER CENT ZINC AND FROM 0.4 TO 1 PER CENT OF ZIRCONIUM, THE ZIRCONIUM IN SAID ALLOY DISSOLVING IN HYDROCHLOIC ACID ON SUBJECTING A SAMPLE OF THE ALLOY TO THE DISSOLVING ACTION OF A 20 PERCENT SOLUTION OF AQUEOUS HYDROCHLORIC ACID. 